This invention relates to the treatment of articles by subjecting them to a mixture of ethylene oxide and an inert gas and, more specifically, to the use of such mixture for an indefinite number of successive treatment cycles. While various treatments with ethylene oxide are known, e.g. decontamination of spices, the most common one, and the treatment to which this invention is particularly directed, is that of sterilization. Accordingly, the invention will be described in detail hereinafter for purposes of illustration by reference to the art of sterilization with ethylene oxide.
Ethylene oxide gas is widely used as a sterilizing agent in spite of its known problems of flammability, as described in Industrial and Engineering Chemistry, June 1950, at pages 1251-1258. For this reason, it is usually mixed with an inert gas, such as carbon dioxide, nitrogen or one of the halogenated hydrocarbons, in a proportion of about 10% to perhaps 30% of ethylene oxide.
By reason of the cost of such mixtures, it is desirable to reuse them for as many cycles as possible, as is discussed in Satus U.S. Pat. No. 3,372,980; Ernst U.S. Pat. No. 3, 549,312 and Skocypec et al U.S. Pat. No. 3,989,461.
However, with multiple sterilization cycles, the mixture acquires a small amount of air each time it is reused, so that the increasing proportion of air in the mixture after a number of reuse cycles produces a mixture which approaches the region of flammability. Satus solves the problem by venting the entire mixture before reaching the region of flammability and replacing it with an air-free mixture. Although this is effective from the standpoint of safety, it is expensive in its utilization of gas mixtures. Ernst and Skocypec et al both use fluorinated hydrocarbons as their inert gas for reuse, the air being separated during the condensation. These systems have the disadvantage of necessitating the use of relatively expensive inert gases which can be condensed to a liquid at relatively high temperatures. They are not economically feasible for low temperature condensing gases such as carbon dioxide and nitrogen which cannot be so readily condensed.
U.S Pat. No. 4,130,393 issued to Fox relates to an improvement in procedure for sterilization with and the recycling of ethylene oxide and has for its primary task to provide a method of reuse of a mixture of ethylene oxide and an inert gas for an indefinite number of successive sterilization cycles without condensing the mixture.
In accordance with the invention described and claimed in the aforementioned patent, this task is solved by providing a procedure for sterilizing successive loads with a nonflammable gas mixture containing at least 10 percent ethylene oxide and an inert gas wherein the gas mixture is recirculated to a storage tank after each sterilization cycle while continuously maintaining it as a gas and refortifying it by the addition of substantially pure ethylene oxide to maintain a percentage of ethylene oxide of at least 10 percent for each sterilization cycle, the mixture acquiring a small amount of air during each sterilization cycle for which it is used, the refortification procedure with ethylene oxide and the acquisition of air resulting from each successive sterilization cycle being characterized as having at least 10 percent ethylene oxide, a decreasing proportion of inert gas and an increasing proportion of air; periodically venting only a fraction of this mixture whenever the increasing proportion of air produces a mixture approaching flammability and increasing the proportion of inert gas in the mixture to reduce the proportion of air while retaining the proportion of ethylene oxide at least 10 percent; and thereafter continuing sterilizing successive loads and repeating the above procedures prior to each such successive sterilization.
The foregoing process described in U.S. Pat. No. 4,130,393 constitutes a significant improvement in the recovery and reuse of the expensive ethylene oxide sterilizing gas while precluding the sterilizing gas mixture from achieving percentages of air in the mixture from being sufficiently great, e.g. on the order of 50 percent, so as to approach the percentage where the mixture is flammable.
Nevertheless, the requirement to periodically vent a proportion of the mixture prior to the next successive batch sterilization does present certain significant disadvantages inherent in the venting of the ethylene oxide.
With the process described in the aforementioned Fox patent, it is not possible to recover more than 75 percent of the ethylene oxide, a costly loss of the ethylene oxide.
More importantly, since at least 25 percent of the ethylene oxide is vented, existing safety and health regulations by such regulatory bodies as local OSHA and EPA administrations will not permit plant usage of the system without the use of scrubbers or other ethylene oxide abatement (capture) equipment.
The present invention is directed to improving the recovery level to a significantly higher level, e.g. on the order of that obtainable with the abatement equipment, thereby making the process acceptable for plant use without the need for such expensive equipment.
Accordingly, and stated more simply, the present invention is directed to a significantly more cost-effective procedure in the manufacture of sterile medical or other articles utilizing ethylene oxide as the sterilizing medium.